Horrocruxes

To make a horrocrux you need to murder someone, when you do so your soul splits in two pieces, and you hide a part of your soul in an object, so even if they kill you, you will not die because your soul is still alive in another part of the world. What Voldemort did was not only to murder one person but to murder seven people so his soul got split on seven pieces.

Killing splits the soul because it is a violation against nature.